The use of chiral non-racemic phosphorus compounds for catalytic asymmetric synthesis has grown enormously in the last three decades, such compounds providing many of the most successful ligands for metal-based catalysts (Ojima, 2000; Brunner et al., 1993).
Asymmetric reactions making use of metal catalysts with chiral phosphine ligands include alkene hydrogenations, hydroformylations and hydrosilylations, allylamine isomerisations, allylic substitutions and a number of cross coupling procedures. Some of these processes have gained industrial significance, e.g. Monsanto's L-dopa process (Knowles, 1986); Anic and Monsanto Aspartame process (Kagan, 1988) Syntex naproxen process (Noyori, 1989) and Takasago L-menthol process (Kagan, 1998). Chiral phosphorus compounds have been found to be useful non-metallic catalysts in their own right (Noyori, 1989).
Most of these catalysts involve the use of C-chiral, rather than P-chiral, phosphorus ligands, primarily because they are more easily prepared. However, P-chiral ligands can be of great value in catalytic asymmetric synthesis, as exemplified by the rhodium/diPAMP catalyst, developed by Knowles, which is one of the most successful catalysts used for the L-dopa and Aspartame syntheses.

In light of the beneficial properties of P-chiral phosphorus compounds in asymmetric synthesis, the search for efficient methods for the synthesis of P-chiral, non-racemic phosphines and related compounds such as phosphine oxides and phosphine sulfides continues to be of prime importance (Pietrusiewicz et al., 1994).
A number of strategies have been employed in the synthesis chiral phosphines. In principle, the most direct route to optically active phosphines is to resolve the racemic phosphine by making diastereomeric transition metal complexes. However, problems associated with the separation of the complexes, the synthesis of optically active ligands and the recycling of expensive metals have prevented this method from being generally applied. Another method used to resolve phosphorus compounds is the formation of phosphonium salts using a chiral counterion (Horner et al., 1964). However, this route has a number of limitations, especially in cleavage reactions of the resultant non-racemic salts where the stereochemical outcome cannot be guaranteed (Valentine, 1984).
The generation of chiral phosphines oxides from phosphinate esters has been widely used (Valentine, 1984), but the success of this method heavily depends on the availability of chiral phosphinate esters, and much effort has been expended in the search for methods to generate these esters, with only limited success. Likewise, the synthesis of chiral phosphines by the electrophilic substitution of chiral phosphonites (Valentine, 1984) is hindered by the availability of suitable phosphonites, which have low optical stability compared with phosphines.
Reduction of chiral four coordinated phosphorus compounds such as phosphine oxides is perhaps the most common route to chiral phosphines and can be achieved by a number of reagents including hydrides, boranes and silanes, the choice of which is determined by the sensitivity of the compound to reduction and the stereochemistry required in the product phosphine. At present, the preferred reductants for phosphine oxides are silanes. However, the use of such reduction methods has merely pushed the stereoselectivity problem back to an earlier stage in the synthesis, i.e. a source of a chiral four-co-ordinated phosphorus compound is now required, such as a chiral phosphine oxide. The synthesis of enantiomerically enriched phosphine oxides and phosphine sulfides based on the kinetic resolution of P-chiral three-coordinate phosphorus compounds using pure bis-phosphoryl or bis-thiophosphoryl disulfides is discussed in Perlikowska et al, 2001.
There remains a clear need for methods of preparing P-chiral four-co-ordinated phosphorus compounds. Such compounds can be converted into the corresponding P-chiral three-coordinated phosphorus compounds by reduction and have important uses in pharmaceutical and agrochemical applications in their own right.